1. Field of the Invention
The field of the invention is fiber reinforced composites having an open-truss of lattice structure and methods for their manufacture.
2. Brief Description of the Related Art
Until recently, structures such as frames, trusses, struts, shafts, bridges, etc. were constructed from an assemblage of standard metal elements, mass-produced and commercially available lengths and cross-sectional geometries. The geometries included plates, sheets, angles, channel sections, I-beam shapes, and hollow, circular, square and rectangular tubing (Manual of Steel Construction, 8th Edition, American Institute of Steel Construction, 1987), and are commonly made from steel or aluminum. It is well known that truss structures provide high strength and stiffness while minimizing weight. Metal truss structures are composed of large apex members or beams connected along the length of the beams with smaller connecting elements that are welded or bolted between the beams. The connecting elements are often placed in an X or W configuration along the beams. The advent of high strength fibers has ushered in a new approach to the creation of light weight and strong structural elements.
When producing composite trusses, welding of the connecting elements is not possible, and bolting is not the most desirable joining method for composite materials because of stress concentrations at the bolt holes, which may lead to failure.
In composite materials, a reinforcement material is distributed in a second bonding material called the matrix. The application considered here is concerned with the polymer matrix composites (PMC), where the reinforcement materials are fibers of high strength, and the matrix is a thermosetting or thermoplastic polymer. Most often, the reinforcing fibers comprise one or more of the available high performance fibers such as graphitic carbon, glass, para-aramid (such as Kevlar™), LCP (such as Vectran™) and others. PMC's are known to offer strength and weight advantages over aluminum and steel, but in order to achieve these advantages the structure and elements in the assemblage of the structure must account for the non-isotropic properties and directionality of composite strength. That is, composites achieve their strength if the largest loads and stresses can be directed along the direction of the fibers.
Despite these differences in materials, and the difficulties of qualifying new materials, there is a need for composite truss structures to minimize the weight of construction elements, since a well-designed composite typically offers equivalent strength at weight savings of 20 to 60% over equivalent metal structures. The question then becomes: how to manufacture a composite truss or other open structure composite structure without the need for bolted joints within the truss.
Braiding is a method of forming a seamless “fabric” tube and is often used to form shapes useful as composite structures after resin impregnation and curing. Braided sleeves have a unique ability to form polygons, flat plates, angle brackets, I beams, and indeed—any closed geometry having uniform or even variable dimensions along the length. This sort of shape variation accompanied by resin impregnation or resin infusion and curing to stiffen the resin, makes braiding an ideal method for construction of composite reinforcement.
As mentioned above, a truss structure is the most efficient for strength and stiffness at low weight. The question is can the braided seamless tube be constructed approximating the beams and open lattice support structure that comprise a truss. That idea is the subject of this invention. Of course, to be practical, it is necessary that the structure should be efficiently manufactured, preferably on simple machinery already in use. One objective of this invention is to produce a fiber reinforced open structured truss preform on a conventional maypole braider.
Infusion of large structural elements with resin is difficult, especially when the large structural elements are immediately adjacent to open spaces in a truss structure. Since the beams and cross members of a truss are large, an array of small yarns or tows have been preimpregnated with resin and arranged in a jacketed parallel bundle to make a large diameter (i.e. high filament number) yarn structure. An assembly of small yarns or tows to produce large prepreg yarns is the subject of a copending patent application based on preliminary patent application 61/624,534, filed Apr. 16, 2012, the subject matter of which the present application incorporates by reference in its entirety.